1. Field of the Invention
This invention relates to a process for the preparation of 1,1-dichloro-1,2-difluoroethane (HCFC-132c) from 1,1-dichloroethylene (vinylidene chloride) using a combination of lead dioxide (PbO.sub.2) and anhydrous hydrogen fluoride (HF) as fluorinating agent.
2. Description of Related Art
1,1,2-Trichloro-1,2,2-trifluoroethane (CFC-113) has developed a major market as a halogenated solvent and cleaning agent in the electronics, aerospace, and metal working industries. The recent discovery that chlorofluorocarbons (CFCs) contribute to ozone destruction in the upper atmosphere has led to regulations requiring a phase out of its use. This has created a demand for hydrogen-containing chlorofluorocarbon solvents (HCFCs) which have similar properties to CFC-113 but are unstable enough in the atmosphere to largely decompose before reaching the stratosphere. One of these potential replacements is 1,1-dichloro-1,2-difluoroethane (HCFC-132c). The structure of HCFC-132c suggests low toxicity and preliminary tests indicate this compound is an excellent solvent with an appropriate boiling point (48.degree. C.).
One known method for preparing vicinal difluoro compounds is by the addition of fluorine (F.sub.2) to the double bond of the corresponding acyclic olefin. While elemental fluorine itself may be used, the high heat of reaction tends to promote undesired side reactions which can result in a reduced yield of the desired F.sub.2 addition product. Instead, compounds capable of delivering fluorine, such as high valency metal fluorides, have been used and in certain cases have provided a more controllable reaction. For example, U.S. Pat. No. 2,466,189 discloses the process of adding fluorine to the double bond of a variety of olefins using a mixture of lead dioxide and HF. This work is also reported in an article by Henne and Waalkes (J. Amer. Chem. Soc. 67, 1639-1640 (1945)) in which it is surmised that the lead dioxide and HF function by generating nascent lead tetrafluoride, which decomposes into lead difluoride and F.sub.2 to be accepted by the olefin. The reaction was generally carried out by adding the olefin to the lead dioxide, cooling the mixture to -20.degree. C. or below, adding the HF, allowing the temperature to rise to 80.degree. C. to 100.degree. C., and separating the resulting material. The importance of the exact procedure for preparing the lead tetrafluoride was illustrated by experiments in which the lead dioxide was added after the olefin and HF were mixed. In this case no fluorination occurred. The Henne work was primarily focused on fully halogenated compounds. When this procedure was applied to 1,2-dichloroethylene, the yield of the addition product 1,2-dichloro-1,2 difluoroethane was the poorest of all those reported, only 17% of theory. It was not tried on the fluorination of 1,1-dichloroethylene, nor was this important and well-known olefin listed among the olefins representative of those to which the process could be applied.
Rausch et al. (J. Org. Chem. 28, 494 (1963)) extended the above work to other metal fluorides such as cobalt trifluoride, manganese trifluoride, silver difluoride, and cerium tetrafluoride. In an experiment to fluorinate 1,2-dichloroethylene with cobalt trifluoride at a reaction temperature of about 35.degree. C., the yield of 1,2-dichloro-1,2-difluoroethane was 53% of theory compared to the yield of 17% with lead tetrafluoride. In a similar experiment using cobalt trifluoride to fluorinate 1,1-difluoroethylene at a reaction temperature of 125.degree. C., the yield of 1,1,1,2-tetrafluoroethane ranged from 54% to 81% of theory. Nearly all the olefins tested gave at least some of the desired addition product. Surprisingly, when the same experiment was repeated on 1,1-dichloroethylene at a reaction temperature of 35.degree. C., they found none of the desired addition product. Instead a variety of products were found, all of which were partially dehydrohalogenated. The authors concluded, "The failure of 1,1-dichloroethylene to produce the addition compound may be attributed to its lack of stability."
In view of the above and not withstanding the presence of a catch-all phrase in the Waalkes patent that other acyclic olefinic compounds may be employed, the data presented in this patent and corresponding publication by Henne et al. cited in the patent establishes that for the series of perchloroethylene, trichloroethylene, and 1,2-dichloroethylene diminishing yields of symmetric addition of fluorine across the double bond is observed. Furthermore, Henne et al. admits that addition of fluorine to the double bond has been observed in only a few cases and is very impractical because of the large amount of heat evolved breaks down the organic material. The above in combination with the observation by Rausch et al. that fluorinating 1,2-dichloroethylene using CoF.sub.3 or PbF.sub.4 to produces symmetric addition and when 1,1-dichloroethylene was fluorinated no symmetric addition occurred but instead dehydrohalogenation occurred exclusively leads one skilled in the art to conclude that 1,1-dichloroethylene will not under go symmetric addition of fluorine but will dehydrohalogenate.
The first successful attempt to produce HCFC-132c was a laboratory preparation reported by Bissell and Fields (J. Org. Chem. 29, 1591 (1964)). They used a mixture of lead dioxide and sulfur tetrafluoride (SF.sub.4) in a 25 ml reactor to add fluorine to the double bond of a variety of halogenated olefins including 1,1-dichloroethylene. In a single run, they reported a yield of 1,1-dichloro-1,2-difluoroethane of 59% of theory, with a reaction temperature varying from below 0.degree. C. to 100.degree. C. in the course of the reaction. In a broader set of experiments on the fluorination of trichloroethylene, they concluded that preformed lead tetrafluoride gave very low yields and conversions. This indicates that the above results with SF.sub.4 and PbO.sub.2 cannot be extrapolated to PbF.sub.4 made by other means. However, SF.sub.4 is a highly toxic material with inhalation toxicity comparable to phosgene. In addition it is a very expensive chemical costing about 50 times as much as the HF and it is available only in small high pressure cylinders. Thus, SF.sub.4 is used primarily as a laboratory reagent, with very limited commercial applications and is not presently considered to be a practical means of preparing HCFC-132c. Consequently none of the above references teach a process suitable for preparing 1,1-dichloro-1,2-difluoroethane which could be used with low-priced commercial raw materials.